Commonly, celestial navigation is carried out by employing a sextant which is used to measure at any one time the angle of a heavenly body such as the sun to the horizon. This can be either achieved by the use of marine type sextant which is used to measure the angle relative to the true horizon or a bubble-type sextant which measures the angle relative to an artificially-created horizon. In order to plot a position having measured the angle as above, it is necessary to accurately record and/or calculate the Greenwhich Mean Time at which the sight was taken after which it is necessary to resort to a Nautical Almanac which lists the position or each heavenly body in the sky for each second of each minute of each day of the year and tables such as Sight Reduction Tables to calculate the true position of the vessel or aircraft.
The above common method, whilst in general if accurately carried out enables the position of a craft such as a marine craft or aircraft to be accurately determined, suffers from a number of disadvantages. Firstly, there is the obvious disadvantage that the sextant must be used correctly so as to accurately measure the angle of the heavenly body to the horizon. Secondly, it is necessary to employ an accurate chronometer or other timing means which indicates the exact time at which the sight is taken. Furthermore, there must be a direct and accurate correlation between the time at which the sight is taken and the time noted on the chronometer to ensure an accurate fix. This often requires the use of two persons one who uses the sextant to measure the angle of the heavenly body and the other of whom reads the correct time on being given a signal by the person using the sextant. Having obtained the time and angle readings it is then necessary to resort to the requisite tables and Nautical Almanac to plot the required position. This can be a time consuming process and a process which can be prone to errors.